GB2192901A

GB2192901A - Chemical vapour deposition

Info

Publication number: GB2192901A
Application number: GB08616989A
Authority: GB
Inventors: Bhikhu A Unvala
Original assignee: Unvala Ltd
Current assignee: Unvala Ltd
Priority date: 1986-07-11
Filing date: 1986-07-11
Publication date: 1988-01-27
Also published as: JPS6367728A; GB8705062D0; GB8616989D0

Abstract

Precursor (9) is atomised by being ionised (11,12) and is then vapourised (14) prior to deposition. The ionised vapour is controlled by electrodes (2,15) to control its flow and/or its deposition on a substrate. <IMAGE>

Description

SPECIFICATION Chemical vapour deposition The present invention relates to chemical vapour deposition (CVD). It is concerned for example with Metal-Organic Chemical Vapour Deposition (MOCVD) which is also known as Metal-Organic Vapour Phase Epitaxy (MOVPE).

An illustrative application of the invention is the manufacture of integrated circuits.

Chemical Vapour Deposition is a known technique for depositing a layer of material onto a substrate. A precursor in liquid form, e.g. dissolved in a solvent, or in the form of a suspension or emulsion is vapourised, and the vapour flows to a chamber containing the substrate, where (some of) the vapour of precursor deposits on the substrate. The substrate is heated and the precursor decomposes to leave on the substrate a layer of the desired material. In MOCVD, the precursor is a metal organic compound of the desired material, which is advantageous because the chemical bonds of such a compound break easily at a relatively low temperature.

An inert carrier gas, e.g. Nitrogen, may be used to transport the vapour from its source to the substrate. The process may be carried out at atmospheric pressure (High Pressure MOCVD) or at less than atmospheric pressure (Low Pressure MOCVD).

Alternatively, the process may be carried out under conditions of near-vacuum.

According to one aspect of the present invention there is provided a method of chemical vapour deposition in which a vapour of precursor to be deposited on a substrate is ionised. There is also provided apparatus for performing chemical vapour deposition, including means for forming an ionised vapour of precursor to be deposited on a substrate.

It is known to produce vapourised precursor in the following way. The dissolved precursor is contained in a medical hypodermic syringe.

The needle of the syringe extends through a sealed entrance port of the CVD apparatus.

The syringe has a plunger which is driven by an electric stepper motor to introduce a succession of droplets into the apparatus. The droplets are heated in the apparatus to vapourise them. This known method of producing vapourised precursor has the disadvantage that the vapour is produced from a succession of relatively large drops, which are produced, and thus vapourised, at discrete intervals of time, resulting in fluctuations in concentration of the vapour at the substrate.

In accordance with another aspect of the invention, fluctuations of concentration of vapour of precursor are reduced by continuously atomising and vapourising the precursor (whether or not the vapour is ionised).

In an embodiment of the said one aspect of the present invention, the liquid precursor is continously atomised and the said ionised vapour is produced from the atomised precursor.

In another embodiment the liquid precursor is continuously atomised by being ionised and is then vapourised.

By continuously atomising the liquid precursor and vapourising the atomised precursor, concentration fluctuations are reduced because the atomisation process prevents droplets being formed even when a stepper motor is used.

In the known CVD method and apparatus it is difficult to control the vapour. According to a further aspect of the invention a force is applied to the vapour to control its flow and/or its deposition.

According to an embodiment of the said one aspect of the present invention, an electric field is used to control the ionised vapour of precursor. Alternatively a magnetic field is used to control the ionised vapour of precursor.

In the known CVD method a carrier gas is used which flows through the CVD apparatus resulting in a relatively high loss of the vapour which is swept out of the chamber. It is also difficult to control the flow of the vapour to the chamber and to control the deposition of the vapour on the substrate. Furthermore, when it is desired to introduce a different precursor into the apparatus it is necessary to wait until the previous precursor has been flushed out of the apparatus.

In another embodiment of the said one aspect of the invention, a substrate control potential is applied to the substrate onto which the vapour is to be deposited, to control the deposition of the ionised vapour on the substrate. The flow control potential may be selected to substantially prevent the vapourised precursor from reaching the substrate. Thus, with the use of ionised precursor, it is possible using the flow control electrode to control the flow of ionised vapour. By applying a blocking potential to the electrode, flow of ionised vapour may be stopped quickly, without waiting for the vapour to be flushed out of the apparatus. The ionised vapour of another precusor can then be introduced into a chamber containing the substrate.

In yet another embodiment of said one aspect a substrate control potential is applied to the substrate onto which the vapour is to be deposited to control the deposition of the ionised vapour on the substrate.

Thus, with the use of ionised precursor it is possible to control the deposition of the ionised precursor onto the substrate electrically.

By applying to the substrate a potential which attracts the ionised vapour to the substrate loss of precursor from the apparatus may be reduced.

For a better understanding of the present invention, reference will now be made by way of example to the accompanying drawings in which: Figure 1 is a schematic diagram (which is not to scale) of an illustrative example of a CVD apparatus in accordance with the invention, Figure 2 is schematic diagram (which is not to scale) showing a modification of the apparatus of Fig. 1, and Figure 3 is a schematic diagram (which is not to scale) of a further CVD apparatus in accordance with the invention.

The CVD apparatus shown in Fig. 1 comprises a quartz chamber 1 containing a substrate 2 onto which a desired material is to be deposited. An inert carrier gas such as Nitrogen or Argon flows from a source (not shown) via a vapour flow path 3 into the chamber 1 and then out of the chamber. A source 4a having an exit vapour path connecting to the path 3 provides to the path 3 a vapour of precursor. The vapour of precursor is carried by the carrier gas to the chamber 1 where it is deposited on the substrate 2. The substrate is heated by a heater 5, which in this example comprises an infra red heating element 5a in an elliptical reflector 5b. The precursor thermally decomposes on the substrate to leave a layer of the desired material on the substrate 2.

In Fig. 1, a further source 4b may be used to provide another precusor, also vapourised.

The apparatus may be operated at atmospheric pressure or at less then atmospheric pressure.

The precursor is a compound of the desired material, which decomposes to form the desired material. Examples of precursors and desired materials are known in the art: for example all Halides of the materials to be deposited.

The precursor may be a metal-organic compound of the desired material, such as are known in the art. For deposition of an oxide the precursor is an oxygen containing Metalorganic compound in particular a Metal Alkoxide. For deposition of a Metal the precursor is a Metal Alkyl. For a semiconductor many compounds are known in the art; e.g.

for Gallium Arsenide the precursor is TMGa AsH3 or TE . Ga-AsaH3 where TM is trimethyl and TE is triethyl. In addition adducts of the form Trimethylindium-Trimethylamine adduct and Trimethylindium-Trimethylphosphive adduct may be used.

The substrate may be a semiconductor substrate as used in the manufacture of integrated circuits. For example it may be of silicon, germanium or a group Ill-V compound such as Gallium Arsenide.

The material deposited on the substrate may be a semiconductor, a metal, a metal oxide, e.g. Al203, TiO a metal nitride, an oxide or nitride of silicon e.g. SiO, SiO2 or Si3N4.

The material may also be a material such as Lead Titanate or Lithium Niobate which is an electro-acoustic material used in for example surface acoustic wave devices.

The source 4a (or 4b) comprises a sealed entry port 6 through which the needle of a medical hypodermic syringe extends. The syringe 8 contains the precursor in liquid form 9. For example the precursor may be in the form of a suspension or emulsion. Aiternatively the precursor may be dissolved in a solvent. The solvent is an organic material of very low water content and with high volatility as is known in the art, e.g. cyclohexane or benzene. A drive mechanism 10 such as stepper motor moves a plunger of the syringe to deliver the dissolved precursor. Alternatively gravity feed could be used.

In accordance with the present invention, the precursor delivered from the syringe is continuously formed into an ionised vapour.

In the example shown in Fig. 1, the needle 7 is earthed. An electrostatic induction electrode 11 which is held in a PTFE holder 111 surrounds the glass tube of the entry port 6, and the needle 7. The electrode 11 is energised by a potential supply 12 to minus 10kV or more to atomise the precursor by positively ionising it, as it is delivered from the needle.

The atomised ionised precursor tends to move away from the needle and spread out due to the mutual repulsion of the like positive charges.

A further electrode 13 in the holder is energised to plus 10kV or more to compress the body of atomised ionised precursor.

The precursor is then heated by heaters 14 to vapourise it. The vapour may be heated to a temperature in the range 70"C to 300"C or even higher depending on the precursor and the liquid in which it is dissolved or suspended or with which it forms an emulsion. If the solvent is cylohexane the temperature is about 150"C. The heaters are electrical resistance heaters in the examples shown in the Figures. Other suitable heaters may be used.

The vapourisd and ionised precursor then flows through the vapour path at the exit of the source into the path 3. The ionised vapour is then carried by the carrier gas along the flow path 3 into the chamber 1.

A flow control electrode 15 is provided in the path 3. It may be for example a conductive mesh extending across the path. By suitably controlling the potential applied to the electrode by a source 16, the flow of the ionised vapour past the electrode 15 is controllable. By applying to the electrode a suitable voltage having the same polarity as the ions produced by the source 4a, e.g. plus 10kV or more in this example, the positive ions are repelled, stopping their flow to the chamber. Repulsion is preferred because the ions are not deposited on the electrode.

Alternatively by connecting the flow control electrode 15 to ground, or to a negative potential, the positively charged ions of the va pour supplied by source 4a are attracted to the electrode, also with the result that the flow is stopped. However, that has the possible disadvantage that ions which are deposited on the electrode may be released later when their presence in the chamber 1 is not desired.

Thus the flow of ions from the source 4a to the chamber 1 may be stopped, started, and controlled by the use of the flow control electrode.

The substrate 2 is supported on an electrode which may have a control potential applied to it from a source 17 to control the deposition of the ionised vapour on the substrate. For example by applying a negative potential to the electrode and thus the substrate, the positive ions from the source 4a are attracted to the substrate. That has the effect of reducing the amount of vapour of precursor which would otherwise be carried out of the chamber by the carrier gas. Also the amount of precursor which needs to be used may be reduced. Thus there will be less contamination of the interior surfaces of the chamber 1 by precursor.

By applying a suitable positive voltage to the substrate the positive ions may be repelled from the substrate.

As shown in Fig. 1, there are two sources 4a, 4b of ionised vapourised precursor. As described above, source 4a is used and it produces positive ions and flow control is performed only using the electrode 1 5 near the entrance of the chamber 1.

In a modification of the apparatus of Fig. 1 as so far described instead of providing one flow control electrode in the path 3, two electrodes 15a, 15b and potential sources 16a, 1 6b may be used in the respective vapour paths at the exits of the sources 4a, 4b to separately and independently control the flow of ions from the sources into the path. In addition the electrode 15 may be used as weli to control the flow of vapour into the chamber 1.

In an alternative arrangement, shown in Fig.

2 the sources 4a and 4b are coupled to the chamber 1 by separate flow paths 3a, 3b.

Each path is provided with its own electrode 15a, 15b and potential supply 16a, 16b. Thus the flow of ionised vapour from the sources 4a, 4b is separately and independently controllable.

The sources 4a,4b could be operated to produce positive and negative ions respectively. The potential applied to the substrate may be controlled so as to repel the positive ions from source 4a and attract the negative ions from source 4b and vice/versa.

Referring to Fig. 3 yet another CVD apparatus is shown. The apparatus has a source 4 of vapourised ions of precursor. The source includes a hypodermic syringe having a plunger driven by a suitable drive (not shown).

Precursor delivered by an earthed needle 7 is atomised by being ionised. For that purpose an electrode 11 held in a PTFE holder 111 is energised to 10kV or more. The atomised and ionised precursor is vapourised by a heat source 14.

In the apparatus of Fig. 3, a set of electrodes 30 are provided for forming a focussed beam 31 of ions which is directed onto, and may be scanned across, a substrate 2. The electrodes are individually energised by suitable energising means 32, via respective connections.

The set 30 of electrodes comprises electrodes 30a and 30b for focussing and accelerating the ions of precursor, and two pairs of electrodes 30c, 30d for scanning the focussed beam of ions across the substrate. The beam of ions is controlled to selectively deposit the precursor where required on the substrate 2 by scanning the beam path across the substrate and cutting off the beam as necessary.

In the apparatus of Fig. 3, the set of electrodes 30 may be replaced by magnetic coils for magnetically forming the beam and scanning it across the substrate.

Various modifications may be made to the above described embodiments. For example instead of directly atomising the precursor by ionising it alternatives may be used.

In one such alternative, the precursor delivered by the needle is ultrasonically atomised by for example a piezo-electric device attached to the needle.

The atomised precursor may then be ionised and then vapourised as described hereinbefore. In another alternative the atomised precursor is vapourised before being onised.

The chamber 1 may be of stainless steel or Aluminium (or any other material known in the art to be suitable) instead of quartz.

The solvent in which the precursor is dissolved may be Methanol for these precursors for which it is suitable as known in the art.

The drive mechanism 10 may be a precision liquid dispenser which operates by producing controlled pulses of air pressure to displace liquid from a disposable syringe.

Instead of using an induction electrode 11 energised to a high voltage of opposite polarity to the ions to be produced and earthing the needle 7, the needle may be energised to a high potential with the same poiarity as the ions to be produced to produce the ions by direct ionisation. That however is disadvantageous because the syringe and its contents are not at earth potential.

Although the invention has been described with the illustrative example of positive ions, negative ions may be used.

Claims

1. A method of chemical vapour deposition in which a vapour of precursor to be deposited on a substrate is ionised.

2. A method according to claim 1 wherein a liquid precursor is continuously atomised and the said ionised vapour is produced from the atomised precursor.

3. A method according to claim 2 wherein the atomised precursor is vapourised and the vapour is then ionised.

4. A method according to claim 2 wherein the atomised precursor is ionised and the ionised, atomised precursor is then vapourised.

5. A method according to claim 1 wherein the liquid precursor is continuously atomised by being ionised and is then vapourised.

6. A method according to any preceding claim wherein an electric field is used to con- trol the ionised vapour of precursor.

7. A method according to any one of claims 1 to 5 wherein a magnetic field is used to control the ionised vapour of precursor.

8. A method according to claim 6 wherein a deposition control potential is used to con trni the deposition of the ionised vapour on the substrate.

9. A method according to claim 8 wherein a deposition control potential is applied to the substrate, onto which the vapour is to be deposited, to control the deposition of the ionised vapour on the substrate.

10. A method according to claim 8 or 9 wherein the deposition control potential is selected to attract the vapour to the substrate.

11. A method according to any one of claims 1 to 6, 8, 9 or 10 wherein a flow control electrode is placed in a path between a source of the ionised vapour and the substrate, and a flow control potential is applied to the electrode to control the passage of the vapourised precursor along the path.

12. A method according to claim 11 wherein the flow control potential is selected to substantially prevent the vapourised precursor from reaching the substrate.

13. A method according to claim 6 or 7 wherein the vapourised ionised precursor is formed into a beam which is focussed onto the target.

14. A method according to claim 13 wherein the focussed beam of precursor is directed to a predetermined portion of the substrate.

15. A method of chemical vapour deposition in which: a plurality of sources of precursor produce respective ionised vapours of precursor, a pluraliy of vapour paths coupled the respective sources to a common chamber containing a substrate onto which the ionised vapours are to be deposited, and flow control potentials are applied to the flow control electrodes in the respective vapour paths to control the flow of the ionised vapours from the sources to the chamber.

16. Apparatus for performing chemical vapour deposition, including means for forming an ionised vapour of precursor to be desposited on a substrate.

17. Apparatus according to claim 15 wherein the forming means comprises means for atomising liquid precursor, and means for ionising and vapourising the atomised precursor.

18. Apparatus according to claim 16 wherein the means for ionising and vapourising comprises means for vapourising the atomised precursor, and means for ionising the vapour.

19. Apparatus according to claim 17 wherein the means for ionising and vapourising comprises means for ionising the atomised precursor and means for vapourising the ionised, atomised precursor.

20. Apparatus according to claim 16 wherein the forming means comprises means for atomising liquid precursor by ionising it, and means for vapourising the ionised atomised precursor.

21. Apparatus according to any one of claims 16 to 20 comprising means for producing an electric field and arranged to control the ionised vapour of precursor.

22. Apparatus according to any one of claims 16 to 20 comprising means for producing a magnetic field and arranged to control the- ionised vapour of precursor.

23. Apparatus according to claim 21 comprising means for producing a deposition control potential for controlling the deposition of the ionised vapour on the substrate.

24. Apparatus according to claim 23 comprising means for applying a deposition control potential to the substrate onto which the chemical is to be deposited, to control the deposition of the ionised vapour on the substrate.

25. Apparatus according to claim 21 or 23 or 24 wherein a flow control electrode is provided in a vapour flow path between the forming means and the substrate and means are provided for applying a flow control potential to the electrode to control the flow of the ionised vapour from the forming means to the substrate.

26. Apparatus according to claim 21 or 22 comprising means for forming the ionised vapour into a beam and for focussing the beam onto the substrate.

27. Apparatus according to claim 26 comprising means for directing the beam onto a predetermined portion of the substrate.

28. Apparatus for performing chemical vapour deposition comprising: a plurality of sources, each of which produces an ionised vapour of precursor, a chamber for containing a substrate onto which ionised vapour is to be deposited, a plurality of vapour paths coupling the respective sources to the chamber, a flow control electrode in each path, and means connected to each electrode, for applying a flow control potential thereto for control ling the flow of the ionised vapour in its associated path.

29. A method of chemical vapour deposition substantially as hereinbefore described with reference to Fig. 1, Fig. 2 or Fig. 3.

30. Apparatus for performing chemical vapour deposition substantially as hereinbefore described with reference to Fig. 1, Fig. 2 or Fig. 3.

31. A method of metal organic chemical vapour deposition according to any one of claims 1 to 14 and 29.

32. Apparatus for performing metal organic chemical vapour deposition according to any one of claims 16 to 28 and 30.

33. A method of chemical vapour deposition in which precursor is atomised and vapourised prior to deposition.

34. A method of chemical vapour deposition in which a force is applied to a vapour of precursor to control its flow and/or deposition.

35. Apparatus for performing chemical vapour deposition including a source of vapour of precursor to be deposited, the source including means for atomising and vapourising the precursor.

36. Apparatus for performing chemical vapour deposition including means for applying a force to the vapour to control its flow and/or deposition.